1. Field of the Invention
The invention relates to a molten carbonate fuel cell which directly converts chemical energy of fuel into electric energy.
2. Description of the Related Art
As illustrated in FIG. 1, a molten carbonate fuel cell generally includes cells 4, and separators 5 sandwiching the cell 4 therebetween. The cell 4 includes an anode or a fuel electrode 2, a cathode or an air electrode 3, and a thin planar electrolytic plate 1 sandwiched between the fuel and air electrodes 2 and 3. A single cell 4 could provide a low voltage, specifically about 0.8 V. Hence, many cells 4 are piled with the separators 5 being sandwiched between the cells 4, to thereby generate a higher voltage.
Anode gas containing hydrogen is supplied to the fuel electrode 2, and cathode gas containing oxygen is supplied to the air electrode 3 through manifolds (not illustrated). Then, chemical reaction is made to occur at each of the cells 4 at high temperature, specifically at about 650.degree. C. to thereby generate electricity, which is taken out in a direction perpendicular to a plane of each of the cells (namely, a vertical direction in FIG. 1). Electrolyte (molten carbonate) contained in the electrolytic plate 1 is molten at the above mentioned temperature, and wetting between the molten electrolytic plate 1 and the separator 5 ensures gas sealing between the separators 5.
FIG. 2 schematically illustrates a generating set including the above mentioned fuel cell for generating electricity using natural gas as fuel. The illustrated generating set includes a molten carbonate fuel cell 6 (hereinafter, referred to simply as a fuel cell) including a plurality of the cells 4 piled one on another, a reformer 7 using natural gas as fuel, a gas blower 8a, and an air compressor 8b. Natural gas is reformed in a reforming tube in the reformer 7 into anode gas containing hydrogen, and the thus reformed anode gas is supplied to an anode of the fuel cell 6 for generating electricity. Exhaust gas exhausted out of an anode of the fuel cell 6 is burned in a combustion chamber of the reformer 7 to thereby reform natural gas contained in the reforming tube, and resulting exhaust gas is mixed with cathode gas containing oxygen. The gas blower 8a is called a carbon dioxide gas recycling blower, because it supplies carbon dioxide (CO.sub.2) produced at an anode to the air electrode.
As illustrated in FIG. 2, anode gas (fuel gas or reformed gas) is supplied into a fuel electrode reaction chamber of the fuel cell 6, and make electrode reaction (anode reaction) at the fuel electrode 2. The resulting exhaust gas is burned, and then supplied into an air electrode reaction chamber together with air. In the air electrode 3, the exhaust gas mixed with air make electrode reaction (cathode reaction), and at the same time remove heat generated in the fuel cell 6. As a result, there is produced high temperature exhaust gas, from which energy is recovered. The above mentioned fuel electrode 2, electrolytic plate 1, and air electrode 3 keep in sufficient contact with one another for providing cell performances. The electrolytic plate 1 is porous so as to sufficiently retain electrolyte therein to thereby prevent occurrence of cross leakage.
FIG. 3 shows an example of gas composition at inlet and outlet ports of each of the cells 4 of the fuel cell 6. In a conventional fuel cell, anode gas is supplied into a fuel electrode reaction chamber through an inlet port (which is located at the left end in FIG. 3), and flows along the fuel electrode 2, while which hydrogen and carbon monoxide (CO) are consumed in anode reaction. Finally, unreacted gas together with reaction products such as CO.sub.2 and H.sub.2 O is exhausted through an outlet port (which is located at the right end in FIG. 3) of the reaction chamber. Thus, atmospheric fuel concentration at the fuel electrode 2 is decreased with fuel consumption between the inlet and outlet ports, and the thus decreased fuel concentration in turn reduces electro motive voltage. The reduction in electro motive voltage caused by reduction in a fuel concentration is called Nernst loss. A problem is that the greater a fuel utilization rate is, the greater Nernst loss is.
Since carbon dioxide gas is necessary for cathode reaction in the air electrode 3, anode exhaust gas is mixed with air supplied from the air compressor 8b, as mentioned earlier. However, since gas to be supplied to the air electrode is mixed with combustion exhaust gas, the concentration of oxygen and carbon dioxide gas is lower than required in the air electrode, which poses a problem of great Nernst loss, similarly to the fuel electrode.
That is, a conventional molten carbonate fuel cell has a problem that the reduction in cell performances caused by Nernst loss is unavoidable, and thus it is difficult to use fuel with a high utilization rate.
In addition, a generating set including conventional molten carbonate fuel cells therein needs a carbon dioxide gas recycling system for supplying carbon dioxide gas generated at a fuel electrode to an air electrode, as mentioned above. This is accompanied by a problem that load response speed is restricted during transition time in which electricity generation output varies. That is, though an amount of carbon dioxide gas generated at a fuel electrode due to electricity generation is equal to an amount of consumption of carbon dioxide gas at an air electrode, it is quite difficult to operate a generating set in accordance with load fluctuation, because of delay in carbon dioxide gas recycle which is caused by the tube capacity in transition.
In addition, since a generating set including conventional molten carbonate fuel cells therein needs a carbon dioxide gas recycling system therefor, there is a necessity of tubes and blowers, which in turn is accompanied by problems that a generating set cannot avoid having a more complicated structure and costs for fabrication of a generating set are consequently increased.